ENH: Allow numba aggregations to return non-float64 results (#53444)

* ENH: non float64 result support in numba groupby

* refactor & simplify

* fix CI

* maybe green?

* skip unsupported ops in other bench as well

* updates from code review

* remove commented code

* update whatsnew

* debug benchmarks

* Skip min/max benchmarks

asv_bench/benchmarks/groupby.py

@@ -57,6 +57,38 @@
     },
 }
 
+# These aggregations don't have a kernel implemented for them yet
+_numba_unsupported_methods = [
+    "all",
+    "any",
+    "bfill",
+    "count",
+    "cumcount",
+    "cummax",
+    "cummin",
+    "cumprod",
+    "cumsum",
+    "describe",
+    "diff",
+    "ffill",
+    "first",
+    "head",
+    "last",
+    "median",
+    "nunique",
+    "pct_change",
+    "prod",
+    "quantile",
+    "rank",
+    "sem",
+    "shift",
+    "size",
+    "skew",
+    "tail",
+    "unique",
+    "value_counts",
+]
+
 
 class ApplyDictReturn:
     def setup(self):
@@ -453,9 +485,10 @@ class GroupByMethods:
         ],
         ["direct", "transformation"],
         [1, 5],
+        ["cython", "numba"],
     ]
 
-    def setup(self, dtype, method, application, ncols):
+    def setup(self, dtype, method, application, ncols, engine):
         if method in method_blocklist.get(dtype, {}):
             raise NotImplementedError  # skip benchmark
 
@@ -474,6 +507,19 @@ def setup(self, dtype, method, application, ncols):
             # DataFrameGroupBy doesn't have these methods
             raise NotImplementedError
 
+        # Numba currently doesn't support
+        # multiple transform functions or strs for transform,
+        # grouping on multiple columns
+        # and we lack kernels for a bunch of methods
+        if (
+            engine == "numba"
+            and method in _numba_unsupported_methods
+            or ncols > 1
+            or application == "transformation"
+            or dtype == "datetime"
+        ):
+            raise NotImplementedError
+
         if method == "describe":
             ngroups = 20
         elif method == "skew":
@@ -505,17 +551,30 @@ def setup(self, dtype, method, application, ncols):
         if len(cols) == 1:
             cols = cols[0]
 
+        # Not everything supports the engine keyword yet
+        kwargs = {}
+        if engine == "numba":
+            kwargs["engine"] = engine
+
         if application == "transformation":
-            self.as_group_method = lambda: df.groupby("key")[cols].transform(method)
-            self.as_field_method = lambda: df.groupby(cols)["key"].transform(method)
+            self.as_group_method = lambda: df.groupby("key")[cols].transform(
+                method, **kwargs
+            )
+            self.as_field_method = lambda: df.groupby(cols)["key"].transform(
+                method, **kwargs
+            )
         else:
-            self.as_group_method = getattr(df.groupby("key")[cols], method)
-            self.as_field_method = getattr(df.groupby(cols)["key"], method)
+            self.as_group_method = partial(
+                getattr(df.groupby("key")[cols], method), **kwargs
+            )
+            self.as_field_method = partial(
+                getattr(df.groupby(cols)["key"], method), **kwargs
+            )
 
-    def time_dtype_as_group(self, dtype, method, application, ncols):
+    def time_dtype_as_group(self, dtype, method, application, ncols, engine):
         self.as_group_method()
 
-    def time_dtype_as_field(self, dtype, method, application, ncols):
+    def time_dtype_as_field(self, dtype, method, application, ncols, engine):
         self.as_field_method()
 
 
@@ -532,8 +591,12 @@ class GroupByCythonAgg:
         [
             "sum",
             "prod",
-            "min",
-            "max",
+            # TODO: uncomment min/max
+            # Currently, min/max implemented very inefficiently
+            # because it re-uses the Window min/max kernel
+            # so it will time out ASVs
+            # "min",
+            # "max",
             "mean",
             "median",
             "var",
@@ -554,6 +617,22 @@ def time_frame_agg(self, dtype, method):
         self.df.groupby("key").agg(method)
 
 
+class GroupByNumbaAgg(GroupByCythonAgg):
+    """
+    Benchmarks specifically targeting our numba aggregation algorithms
+    (using a big enough dataframe with simple key, so a large part of the
+    time is actually spent in the grouped aggregation).
+    """
+
+    def setup(self, dtype, method):
+        if method in _numba_unsupported_methods:
+            raise NotImplementedError
+        super().setup(dtype, method)
+
+    def time_frame_agg(self, dtype, method):
+        self.df.groupby("key").agg(method, engine="numba")
+
+
 class GroupByCythonAggEaDtypes:
     """
     Benchmarks specifically targeting our cython aggregation algorithms

doc/source/whatsnew/v2.1.0.rst

@@ -108,6 +108,7 @@ Other enhancements
 - :meth:`SeriesGroupby.transform` and :meth:`DataFrameGroupby.transform` now support passing in a string as the function for ``engine="numba"`` (:issue:`53579`)
 - Added ``engine_kwargs`` parameter to :meth:`DataFrame.to_excel` (:issue:`53220`)
 - Added a new parameter ``by_row`` to :meth:`Series.apply`. When set to ``False`` the supplied callables will always operate on the whole Series (:issue:`53400`).
+- Groupby aggregations (such as :meth:`DataFrameGroupby.sum`) now can preserve the dtype of the input instead of casting to ``float64`` (:issue:`44952`)
 - Many read/to_* functions, such as :meth:`DataFrame.to_pickle` and :func:`read_csv`, support forwarding compression arguments to lzma.LZMAFile (:issue:`52979`)
 - Performance improvement in :func:`concat` with homogeneous ``np.float64`` or ``np.float32`` dtypes (:issue:`52685`)
 - Performance improvement in :meth:`DataFrame.filter` when ``items`` is given (:issue:`52941`)

pandas/core/_numba/executor.py

@@ -3,6 +3,7 @@
 import functools
 from typing import (
     TYPE_CHECKING,
+    Any,
     Callable,
 )
 
@@ -15,8 +16,86 @@
 
 
 @functools.cache
+def make_looper(func, result_dtype, nopython, nogil, parallel):
+    if TYPE_CHECKING:
+        import numba
+    else:
+        numba = import_optional_dependency("numba")
+
+    @numba.jit(nopython=nopython, nogil=nogil, parallel=parallel)
+    def column_looper(
+        values: np.ndarray,
+        start: np.ndarray,
+        end: np.ndarray,
+        min_periods: int,
+        *args,
+    ):
+        result = np.empty((values.shape[0], len(start)), dtype=result_dtype)
+        na_positions = {}
+        for i in numba.prange(values.shape[0]):
+            output, na_pos = func(
+                values[i], result_dtype, start, end, min_periods, *args
+            )
+            result[i] = output
+            if len(na_pos) > 0:
+                na_positions[i] = np.array(na_pos)
+        return result, na_positions
+
+    return column_looper
+
+
+default_dtype_mapping: dict[np.dtype, Any] = {
+    np.dtype("int8"): np.int64,
+    np.dtype("int16"): np.int64,
+    np.dtype("int32"): np.int64,
+    np.dtype("int64"): np.int64,
+    np.dtype("uint8"): np.uint64,
+    np.dtype("uint16"): np.uint64,
+    np.dtype("uint32"): np.uint64,
+    np.dtype("uint64"): np.uint64,
+    np.dtype("float32"): np.float64,
+    np.dtype("float64"): np.float64,
+    np.dtype("complex64"): np.complex128,
+    np.dtype("complex128"): np.complex128,
+}
+
+
+# TODO: Preserve complex dtypes
+
+float_dtype_mapping: dict[np.dtype, Any] = {
+    np.dtype("int8"): np.float64,
+    np.dtype("int16"): np.float64,
+    np.dtype("int32"): np.float64,
+    np.dtype("int64"): np.float64,
+    np.dtype("uint8"): np.float64,
+    np.dtype("uint16"): np.float64,
+    np.dtype("uint32"): np.float64,
+    np.dtype("uint64"): np.float64,
+    np.dtype("float32"): np.float64,
+    np.dtype("float64"): np.float64,
+    np.dtype("complex64"): np.float64,
+    np.dtype("complex128"): np.float64,
+}
+
+identity_dtype_mapping: dict[np.dtype, Any] = {
+    np.dtype("int8"): np.int8,
+    np.dtype("int16"): np.int16,
+    np.dtype("int32"): np.int32,
+    np.dtype("int64"): np.int64,
+    np.dtype("uint8"): np.uint8,
+    np.dtype("uint16"): np.uint16,
+    np.dtype("uint32"): np.uint32,
+    np.dtype("uint64"): np.uint64,
+    np.dtype("float32"): np.float32,
+    np.dtype("float64"): np.float64,
+    np.dtype("complex64"): np.complex64,
+    np.dtype("complex128"): np.complex128,
+}
+
+
 def generate_shared_aggregator(
     func: Callable[..., Scalar],
+    dtype_mapping: dict[np.dtype, np.dtype],
     nopython: bool,
     nogil: bool,
     parallel: bool,
@@ -29,6 +108,9 @@ def generate_shared_aggregator(
     ----------
     func : function
         aggregation function to be applied to each column
+    dtype_mapping: dict or None
+        If not None, maps a dtype to a result dtype.
+        Otherwise, will fall back to default mapping.
     nopython : bool
         nopython to be passed into numba.jit
     nogil : bool
@@ -40,22 +122,35 @@ def generate_shared_aggregator(
     -------
     Numba function
     """
-    if TYPE_CHECKING:
-        import numba
-    else:
-        numba = import_optional_dependency("numba")
 
-    @numba.jit(nopython=nopython, nogil=nogil, parallel=parallel)
-    def column_looper(
-        values: np.ndarray,
-        start: np.ndarray,
-        end: np.ndarray,
-        min_periods: int,
-        *args,
-    ):
-        result = np.empty((len(start), values.shape[1]), dtype=np.float64)
-        for i in numba.prange(values.shape[1]):
-            result[:, i] = func(values[:, i], start, end, min_periods, *args)
+    # A wrapper around the looper function,
+    # to dispatch based on dtype since numba is unable to do that in nopython mode
+
+    # It also post-processes the values by inserting nans where number of observations
+    # is less than min_periods
+    # Cannot do this in numba nopython mode
+    # (you'll run into type-unification error when you cast int -> float)
+    def looper_wrapper(values, start, end, min_periods, **kwargs):
+        result_dtype = dtype_mapping[values.dtype]
+        column_looper = make_looper(func, result_dtype, nopython, nogil, parallel)
+        # Need to unpack kwargs since numba only supports *args
+        result, na_positions = column_looper(
+            values, start, end, min_periods, *kwargs.values()
+        )
+        if result.dtype.kind == "i":
+            # Look if na_positions is not empty
+            # If so, convert the whole block
+            # This is OK since int dtype cannot hold nan,
+            # so if min_periods not satisfied for 1 col, it is not satisfied for
+            # all columns at that index
+            for na_pos in na_positions.values():
+                if len(na_pos) > 0:
+                    result = result.astype("float64")
+                    break
+        # TODO: Optimize this
+        for i, na_pos in na_positions.items():
+            if len(na_pos) > 0:
+                result[i, na_pos] = np.nan
         return result
 
-    return column_looper
+    return looper_wrapper

pandas/core/_numba/kernels/mean_.py

@@ -60,10 +60,11 @@ def remove_mean(
 @numba.jit(nopython=True, nogil=True, parallel=False)
 def sliding_mean(
     values: np.ndarray,
+    result_dtype: np.dtype,
     start: np.ndarray,
     end: np.ndarray,
     min_periods: int,
-) -> np.ndarray:
+) -> tuple[np.ndarray, list[int]]:
     N = len(start)
     nobs = 0
     sum_x = 0.0
@@ -75,7 +76,7 @@ def sliding_mean(
         start
     ) and is_monotonic_increasing(end)
 
-    output = np.empty(N, dtype=np.float64)
+    output = np.empty(N, dtype=result_dtype)
 
     for i in range(N):
         s = start[i]
@@ -147,4 +148,8 @@ def sliding_mean(
             neg_ct = 0
             compensation_remove = 0.0
 
-    return output
+    # na_position is empty list since float64 can already hold nans
+    # Do list comprehension, since numba cannot figure out that na_pos is
+    # empty list of ints on its own
+    na_pos = [0 for i in range(0)]
+    return output, na_pos

pandas/core/_numba/kernels/min_max_.py

@@ -15,14 +15,16 @@
 @numba.jit(nopython=True, nogil=True, parallel=False)
 def sliding_min_max(
     values: np.ndarray,
+    result_dtype: np.dtype,
     start: np.ndarray,
     end: np.ndarray,
     min_periods: int,
     is_max: bool,
-) -> np.ndarray:
+) -> tuple[np.ndarray, list[int]]:
     N = len(start)
     nobs = 0
-    output = np.empty(N, dtype=np.float64)
+    output = np.empty(N, dtype=result_dtype)
+    na_pos = []
     # Use deque once numba supports it
     # https://github.com/numba/numba/issues/7417
     Q: list = []
@@ -64,6 +66,9 @@ def sliding_min_max(
         if Q and curr_win_size > 0 and nobs >= min_periods:
             output[i] = values[Q[0]]
         else:
-            output[i] = np.nan
+            if values.dtype.kind != "i":
+                output[i] = np.nan
+            else:
+                na_pos.append(i)
 
-    return output
+    return output, na_pos